During the "Walcheren" mission, find a Kar-98 scoped rifle (or any other sniper rifle) and keep it. When you reach the part where you must destroy the tanks with a Flak 88, switch your weapon to the sniper rifle and man the Flak 88, then press L to aim and go into sniper mode with the 88. You will be able to hit the tanks with a sniper's precision.
To obtain 100.0% accuracy on any level, have the "Unlimited grenades" cheat enabled. When you encounter enemies, toss plenty of grenades at them and hide. When all the grenades have blown up, check to see that no enemies remain. If there are, kill them with a rifle to maintain a high accuracy. After killing about thirty enemies with grenades, you should have 100.0% accuracy and should remain that way. If it does not, repeat the process.